1. Field of the Invention
This present invention relates to reinforced composite materials consisting of fibers and wires in a polymer matrix; and, in particular, to responsive, self-activating materials consisting of prestressed fibers and wires restrained and controlled in a polymer matrix which delays dimensional change and external force; and the processing steps for preparing said materials.
The composites of the present invention are force reservoirs which are not capable of resetting themselves to their original state without external intervention and reprocessing.
For the purposes of this current invention the term "fiber" is defined to mean a slender and greatly elongated shape from natural and synthetic materials which has tensile strength and comprises fiber, thread, fiber bundles, filament bundles, rod, and wire.
2. Description of the Prior Art
Since the advent of high-strength fibers and their incorporation into polymer composites, the focus within the prior art has been on the structural ability of composites to resist external forces. A multitude of fibers, fiber arrangements, and polymer matrix compositions have been utilized for structural composites. Structural composites are inert materials, resisting external loads. Inert structural polymer composites have found use in aerospace and recreational products. Some inert structural polymer composites have been used in force-generating mechanisms such as the composite archery bow. Composites used in the archery bow and in the "Carbon fiber reinforced composite coil spring" of U.S. Pat. No. 4,260,143 by Kliger are utilized as springs, resisting an external force. This present invention concerns composites that generate forces rather than resisting forces. The distinction is similar to that between a spring and a mouse trap. The mouse trap has a restraint and release function in addition to the tensioned spring. In this present invention the polymer matrix is the responsive component and acts to restrain, balance, and control stress that was induced in the fibers during the process for preparing the composite material. Dimensional change or strain, force, energy, and activation method are inherent functions of the composite material of this present invention that are determined prior to fabrication.
In the prior art, prestress has been used to render materials and structures resistant to force loadings. The process of prestressing the fibers and strands within composites is well known in the construction of prestressed and post-tensioned concrete structural members. Prestressed concrete is inert and unresponsive, and the release of prestress has usually proved catastrophic to the structure. This present invention uses prestress to store internal energy which by delayed release is utilized external to the material.
In the prior art Rogers et al. used prestress in a responsive composite with internal heating elements. In Rogers the prestressed Nitinol wire expanded the range of the structural modulus of the composite. Rogers composite did not generate an external force, or a dimensional change at the face of the thermoset matrix. The responsive component of the Rogers composite is the shape memory alloy wire, not the polymer matrix. Rogers describes heating the Nitinol wire by electricity, but the external faces of the Rogers composite are not likely to change dimension in a predictable way because the hot wire will likely lose bond with the thermoset epoxy matrix. Rogers describes the prestressed wire as generating a point load on the face of the polymer, which is an indication that little or no bond stress exists between the prestressed element and the polymer matrix, unlike the current invention.
In related art Sigur in U.S. Pat. No. 5,084,219 uses a heat-expanding material to prestress a fiber overwrap onto a composite tube to make a structural member. Sigur combines composite, prestressed fiber, and heat-responsive foam elements. However, the composite is not the responsive element, the prestressed element is outside the composite, the prestress is never released, and the resulting structure is inert. In contrast this present invention combines responsive and prestressed attributes within a composite to control the release of internal prestress energy for external utility.
The preparation of materials by the rule of this current invention is directly related to the steps by which prestress is induced. This invention teaches two methods of preparing these dynamic polymer composites. The two methods are the prestress prior to envelopment method and the prestress after envelopment method. It will be understood by one skilled in the art that some initial prestress present in the fibers is lost to bring the polymer matrix up to a balanced stress. It will also be understood by one skilled in the art that the external means to induce prestress is dependent upon shape. For example to induce prestress in a hollow shape may require a mandrel and to induce prestress in a linear shape may require opposing restraints.
This invention teaches that the continuous stress transfer in the bond between the fiber and the polymer matrix is necessary to be able to contain and restrain high prestress levels which yield high external utility. It will be understood by one skilled in the art that a high prestress causing loss of bond between the fiber and the polymer matrix may be moderated by coating, saturating, and impregnating the fiber with an intermediate material possessing a higher bond-strength than the polymer selected for the matrix.
In the prior art the stiffness of high-modulus fiber has been used for rigidity of structure and spring action. In one class of embodiments this present invention uses the stiffness of high-modulus fiber, prestressed in bending, controlled and restrained by and within a polymer matrix for shrink actuation. Although other embodiments for this present invention exist for the use of straight fibers and straight fiber bundles, the design versatility of the invention would be compromised without the use of coiled, swedged, crimped and otherwise spring-shaped fiber to extend the potential of the fiber's elongation, and therefore the composite's range of dimensional change and strain. Not all fibers can be easily deformed, particularly the high-modulus fibers which find advantageous use in this present invention. However, even carbon fiber bundles have been made that will provide the elongation required, reference McCullough, Jr. et al. in U.S. Pat. No. 4,837,076 and Kitajima in U.S. Pat. No. 5,183,603. McCullough, Jr. et al. used the coil-like bundles in a composite for impact resistance in U.S. Pat. No. 4,868,038. These coil-like fibers and fiber bundles have been provided in the prior art without having been used for prestress internal to the composite.
In the prior art there are various polymers which respond to temperature, light, moisture, age, or chemical media. Both thermomechanically expanded films and shapes, commonly called "shrink polymers" produce relative weak constriction force and lack a mechanism to control the amount of force. Expanding polymer foams share similar lacks. Douglas in U.S. Pat. No. 4,978,564 combines the elements of heat responsive polymer foam with a composite. He uses a heat-activated expanding foam to deploy a conductive wire surrounded by an uncured composite sheath for use in space. Unlike this present invention, Douglas's composite is not the responsive member. Douglas shapes the uncured composite, Douglas does not prestress it.
The heat-activated smart composites of this present invention share similar functions with expanded polymers sold as heat-shrink tubing. For example heat-shrink Polyolefin tubing is sold by 3M and Cryovac Division of W. R. Grace for use as electrical insulation. When heated above the glass-transition-temperature (Tg) these expanded Polyolefin tubes exert a weak constrictive force as they shrink to cover wire splices. Meltsch in U.S. Pat. No. 4,442,153 presents a shape-memory-polymer cable sleeve with such weak constriction force that it can be held together by "gluing, riveting, sewing" (see column 5 line 7). In contrast heat-shrink tubing constructed from a composite according to the rule of this invention, generates a substantial constriction force when heated due to the prestressed fibers which are released by the heat-softened thermoplastic matrix. When cool the resultant composite splice can be stronger than the original wire or cable, exhibiting structural assembly properties.
In the prior art there were various actuators that used shape memory metal alloys for actuators. An example is by Darin Mckinnis of Johnson Space Center as described in NASA Tech Briefs, March 1993. Mckinnis uses a heating element within a hollow shape-memory metal alloy cylinder composed of nickel and titanium to break shearpins and release a bolt. Shape memory metal alloys have demonstrated the simplicity and reliability of responsive dimensional change in a material used as an actuator. However, no shape memory metal alloy has the time-dependent self-activating function of this present invention.
In the prior art various controllers, such as electronic timers, sensors, and relays have been used to restrain force-generating devices such as springs, pressure reservoirs, pumps, motors, and solenoids to create complex, bulky actuator mechanisms. This present invention provides a material for actuation which may be incorporated into deployment, retraction, contraction, and expansion mechanisms that combine force and control functions in one material.